1. Field of the Invention
This invention relates to a process for preparing hydrogen peroxide from the homogeneously catalyzed reaction of hydrogen and oxygen.
2. Description of the Prior Art
Several methods are presently used to manufacture hydrogen peroxide on a commercial scale: (i) electrolysis of sulfate-containing solutions (e.g., ammonium bisulfate) to form persulfate which is hydrolyzed to recover the sulfate and produce hydrogen peroxide; (ii) oxidation of isopropyl alcohol to yield byproduct acetone and hydrogen peroxide; and (iii) autooxidation of anthraquinone to yield hydrogen peroxide. Disadvantages of these methods include the energy intensive nature of electrolysis processes, and the significant capital and raw material costs associated with the chemical processes.
As an alternative to these techniques, the direct chemical reaction of hydrogen and oxygen to form hydrogen peroxide has been investigated by a number of researchers.
U.S. Pat. No. 4,128,627 issued to Dyer et al discloses a homogeneous catalysis process for synthesizing hydrogen peroxide from hydrogen and oxygen, utilizing a water-insoluble catalyst in a two-phase reaction medium. Preferred catalysts in this homogeneous catalysis system are bis(tri(pentafluorophenyl)phosphine) palladium dichloride, bis(tricyclohexylphosphine) palladium dichloride and bis(triphenylarsine) palladium dichloride.
Processes for preparing hydrogen peroxide from its elements utilizing heterogeneous catalysis systems are disclosed in U.S. Pat. Nos. 4,009,252 issued to Izumi et al; U.S. Pat. No. 4,007,256 issued to Kim et al; U.S. Pat. No. 3,361,533 and U.S. Pat. No. 3,336,122 both issued to Hooper; U.S. Pat. No. 3,433,582 issued to Campbell and British Pat. No. 1,094,804 issued to Campbell.
None of these proposed direct synthesis methods, however, is known to have proven sufficiently efficient and satisfactory for preparing hydrogen peroxide on a commercial basis.
Catalysis systems for the preparation of hydrogen peroxide which employ homogeneous catalysis, i.e., catalyst dissolved in the reaction solvent, are preferred over heterogeneous catalysis i.e., solid phase catalyst, systems. Characteristics of homogeneous catalysis systems are generally mild reaction conditions (less energy intensive), high selectivity based on reactants (minimum reactant costs) and relatively high yields.
Despite such advantages, many prior art transition metal homogeneous catalysts suffer from drawbacks that are a deterent to their commercial use. These adverse characteristics include poor catalyst stability under reaction conditions, limited catalyst solubility in the reaction medium, and low reaction rates for the production of hydrogen peroxide.
The homogeneous catalysts of the present invention are intended to minimize the negative characteristics ordinarily associated with homogeneous catalysis systems.
The method of the present invention utilizes an iridium complex as a homogeneous catalyst, in conjunction with a promoter, to produce hydrogen peroxide. Vaska et al, in Science 93, 7099 (1971), reported that such iridium complexes were among several transition metal complexes which catalytically formed water from molecular hydrogen and oxygen under ambient conditions but did not yield hydrogen peroxide as a byproduct.